1. Field of the Invention
The present invention relates to a method and a device for acoustic determination of the moisture content of a multiple gas component gas mixture and in particular to a method and device employing acoustic velocity related measurements in the determination.
2. Description of the Prior Art
In medical applications involving the supply of a breathing gas to a patient, pressurized air and oxygen are often mixed to attain an essentially binary component (oxygen/nitrogen) breathing gas mixture having a desired therapeutic oxygen content. This mixing is often carried out within a mechanical breathing aid, such as a ventilator, respirator or inhalation anaesthetic delivery system, which is designed to also control the subsequent delivery of the mixed breathing gas to a patient. The composition of the breathing gas requires close monitoring and it is well known to employ an acoustic, typically an ultrasonic, analyzer connectable to or integral with the mechanical breathing aid for this purpose. Additionally, it may be desirable to monitor the moisture content of the breathing gas so as to avoid dehydration of a patient receiving the gas, which may occur if a dry breathing gas were provided.
Known acoustic analyzers generally have an acoustic, typically ultrasonic, arrangement devised to emit acoustic energy into and detect acoustic energy from an acoustic path within the breathing gas, and to generate from this an output signal containing acoustic velocity VS related information. A signal processor receives the acoustic velocity related information and determines therefrom the composition of the binary gas mixture according to the known equation:
                              V          S                =                              (                                                            C                  P                  *                                ⁢                                  R                  M                                ⁢                T                                                              C                  V                  *                                ⁢                                  M                  *                                                      )                                1            2                                              (        1        )            wherein T is the absolute temperature (Kelvin) of the gas; RM is the universal gas constant; and C*P, C*V and M* are respectively the specific heat capacity at constant pressure, the specific heat capacity at constant volume and the molecular weight of the binary breathing gas mixture, and are given by:
                              C          P          *                =                                                            C                P1                            ⁢                              M                1                            ⁢                              x                1                                      +                                          C                P2                            ⁢                              M                2                            ⁢                              x                2                                                                                        M                1                            ⁢                              x                1                                      +                                          M                2                            ⁢                              x                2                                                                        (        2        )                                          C          V          *                =                                                            C                V1                            ⁢                              M                1                            ⁢                              x                1                                      +                                          C                V2                            ⁢                              M                2                            ⁢                              x                2                                                                                        M                1                            ⁢                              x                1                                      +                                          M                2                            ⁢                              x                2                                                                        (        3        )            M*=M1x1+M2x2  (4)
wherein the subscripts 1 and 2 refer to a gas 1 and a gas 2 of the binary gas mixture and xi is the fraction of the respective gas in the mixture so thatx1+x2=1  (5)
The pressurized oxygen source connected to such a breathing aid typically originates from an external supplier and has negligible moisture content. The pressurized air, however, normally is generated on-site and is made available at the breathing aid either from an attendant compressor or from a wall outlet connected to a central compressor located within the medical facility. A problem is that this pressurized air normally contains an unknown, small but significant amount of moisture. This moisture may be considered as a third gas (gas 3), component of the breathing gas mixture and can lead to errors in the determination of the composition of the breathing gas which is based on the equations (1)–(5) above.
Including this third gas in the equations (2)–(5) gives:
                              C          P          *                =                                                            C                P1                            ⁢                              M                1                            ⁢                              x                1                                      +                                          C                P2                            ⁢                              M                2                            ⁢                              x                2                                      +                                          C                P3                            ⁢                              M                3                            ⁢                              x                3                                                                                        M                1                            ⁢                              x                1                                      +                                          M                2                            ⁢                              x                2                                      +                                          M                3                            ⁢                              x                3                                                                        (                  2          ′                )                                          C          V          *                =                                                            C                V1                            ⁢                              M                1                            ⁢                              x                1                                      +                                          C                V2                            ⁢                              M                2                            ⁢                              x                2                                      +                                          C                V3                            ⁢                              M                3                            ⁢                              x                3                                                                                        M                1                            ⁢                              x                1                                      +                                          M                2                            ⁢                              x                2                                      +                                          M                3                            ⁢                              x                3                                                                        (                  3          ′                )            M*=M1x1+M2x2+M3x3  (4′)
andx1+x2+x3=1  (5′)
Since the fraction of moisture, x3, in the mixture changes as the O2 fraction changes and assuming all moisture in the mixture comes from the air (gas 1) then:x3=k(1−x2)  (6′)where k is the moisture fraction of the air before mixing.
In order to reduce these errors it is known to determine the volume fraction (here x3) of moisture (gas 3) within the breathing gas mixture other than by employing the acoustic analyzer, such as by using a known moisture sensor, and then to insert the measured value into the equations (2′)–(6′). The composition of the “binary” gas breathing gas mixture can then be determined using the acoustic analyzer.
The addition of a dedicated moisture sensor adds expense and complexity to the analyzer. To avoid these problems with the inclusion of a moisture sensor, an estimation of a probable value for the volume fraction of moisture can be made without measurement and entered into the equations (2′)–(6′) as a constant for use within the acoustic analyser in the determination of the compositional information.